The present invention relates to the technical field of solar cells, and more particularly to an electrically conductive paste for a front electrode of a solar cell and a preparation method thereof.
Solar energy is an inexhaustible source of clean energy. With the increasing depletion of coal, oil and other non-renewable energy, development and use of solar energy have become a big trend. Use of solar cells is a typical means of using solar energy, and crystalline silicon solar cells for which industrial production has been achieved are one type of solar cells.
As the most important core part of crystalline silicon solar cells, a cell sheet needs to collect and export a current generated under light irradiation, so two electrodes need to be fabricated on a front side and a back side of the cell sheet. Many methods can be used to fabricate the electrodes, among which screen printing and co-sintering are the most commonly used production processes currently.
In a crystalline silicon solar cell, an electrically conductive paste for a front electrode, an electrically conductive paste for a back electrode, and a paste for an aluminum back-surface field are coated on a silicon chip by adopting screen printing, and a front electrode is formed on the front side of the silicon chip through co-sintering.
The co-sintered electrode of a crystalline silicon solar cell is required to have strong adhesion, have no ash falling and no deformation of the silicon chip, and be easy to weld and convenient to collect and export the current generated under light irradiation by means of a wire. Compared with the electrically conductive paste for a back electrode of the silicon solar cell, the electrically conductive paste for a front electrode of a silicon solar cell is required to have the ability to penetrate the silicon nitride antireflective layer.
In the prior art, the electrically conductive paste for the front electrode of a crystalline silicon solar cell is composed of a silver powder, a glass frit, an additive and an organic carrier. The glass frit, as an inorganic adhesive, binds the high-conductivity silver powder and the silicon substrate together, and during co-sintering, the molten glass frit etches and penetrates the silicon nitride antireflective layer, so that a good contact is formed between the silver powder and the silicon substrate. Generally, the glass frit in the paste has the following effects: (1) wetting the metallic powder to promote the sintering of the metallic powder; and (2) etching the antireflective layer to promote the contact of the metal and the silicon surface and ensure the binding effect between the metal and the silicon surface. In order to achieve a good ohmic contact of the metallic powder and the silicon surface, the antireflective layer is required to be etched through but not penetrate into a P—N junction region. In the selection of the glass frit, the composition, softening point, thermal expansion coefficient, wetting properties and amount will affect the physical and chemical changes in the sintering process, thereby affecting the performance of the solar cell. In the sintering process, the glass frit is gradually softened, and within a short process cycle, usually 1 to 2 minutes, part of the softened glass frit remains around the metallic powder and flows, and the other part of the softened glass frit flows to reach the antireflective layer at the bottom and induces a reaction. If the content of the glass frit is low, full contact and complete reaction of the glass frit and the antireflective layer cannot be ensured. If it is ensured that the antireflective layer is completely penetrated, a sufficient amount of the glass frit needs to be added. The higher the amount of the glass frit is, the lower the relative content of the electrically conductive metallic phase is, and the lower the probability of contact of metallic particles is, resulting in serious deterioration of conductivity. If a glass frit with a low softening point such as a softening point of lower than 400° C. is selected to ensure that a sufficient amount of glass frit is deposited on the surface of the antireflective layer in the entire process, and react with the antireflective layer completely. But excessively-early softening of the glass frit can clog the communicating pores in the metallic powder, thereby hindering the effective discharge of the organic carrier.
Presently, a Pb—Si based glass material is widely used as the glass frit in the front electrode paste. At the same time, Pb oxide, Te oxide and other oxides or fluorides are used to go through a series of processes of melting, mixing and quenching, to prepare a Pb—Te—O glass material. However, regardless the use of various glass frit materials, due to restrictions of the physical properties of the glass frit, the above technical problems still exist, resulting in process difficulties with narrow windows in preparation of a suitable glass frit and a subsequent conductive paste. Therefore, improved techniques are desired for the manufacture of an electrically conductive paste for forming front electrodes of semiconductor devices.